The invention relates to an apparatus and method for draining a viscous material from a vessel that contains the material. More specifically, the invention is directed to a valve unit that is mounted on the vessel and designed especially for draining the viscous material.
In the production of a linear low density polyethylene product, a second stage in the process involves running the polymer through a devolatilizer (devo) vessel under full vacuum at temperatures above 200.degree. C. During this phase of the process, small amounts of oxygen can leak into the devo vessel through valves, flanges, thermowells, and other components. The oxygen initiates a crosslinking reaction that produces "gels" in the finished polymer. These gels are high molecular weight polymers that represent undesirable substances in the final product.
The oxygen leak problem is partly solved by reducing the number of connections into the devo vessel. For example, in the present operation the devo vessel is connected directly to a gear pump, or an extruder, which delivers the polymer solution into a pelletizing apparatus through a line that doesn't include isolation valves or drain valves. Occasionally, the gear pump or extruder will have a mechanical failure. If the failure occurs when the level of polymer in the devo vessel is high, it's an extremely difficult and dangerous operation to empty the molten polymer out of the vessel, so the pump or extruder can be repaired or replaced.
In the present devo vessel, the polymer solution is emptied from the vessel through a nozzle and drain valve, which are located in the vessel sidewall just above the pump suction. Inside the conventional nozzle is a dead space where stagnant polymer collects. Very small quantities of oxygen can leak into the nozzle through the drain valve, or the fitting that connects the nozzle to the valve, from outside the vessel. The intruding oxygen can cause some of the polymer solution to crosslink and harden, and thus plug off the nozzle. Unplugging the nozzle is a hazardous and time-consuming job that requires "rodding out" the hardened material with heavy-duty hand tools.